504 Transactions. 



It is therefore highly probable that e and h on the seismograms 

 (of which those given are fair specimens) represent the chief 

 normal and transverse waves, due respectively to elasticity of 

 volume and to elasticity of form. Now, if the rocks were sub- 

 jected to great pressure, as must be the case at a few miles below 

 the surface, the strain would be at a maximum just before yield- 

 ing, and the consequent vibrations (a) would be small and rapid ; 

 as the rocks yielded to the strain and were compressed, larger and 

 slower vibrations (e) would be produced ; distortion of the rocks 

 would also generally take place, giving rise to transverse waves (h). 

 The conditions would be satisfied if, under unequal vertical or 

 lateral pressures in two adjoining portions of the earth's crust, 

 there occurred bending or folding of the strata, accompanied or 

 followed by fracture of the rocks such as give rise to faults ; 

 there would be earthquakes, in fact, whenever any sudden ad- 

 justment took place, whether such adjustment were rapid tilting, 

 the formation of a fracture, the rapid sliding of one rock-face 

 over another, or simply the crushing of rocks under great in- 

 crease of pressure, or, what is most probable, several of these 

 causes operating together. 



Have we any evidence that these conditions have been satis- 

 fied in the case of New Zealand earthquakes ? The great earth- 

 quake of 1855 affords evidence that they have been satisfied in at 

 least one instance. The origin — that is, the moving portion of 

 the earth's crust — was at least as large as is indicated by the oval 

 drawn on the map (Plate LIV). The evidence is very clear that 

 on the north-eastern side of this area the elevation was greatest ; 

 that it diminished towards the middle ; that there was neither 

 elevation nor depression in Porirua Harbour ; and that on the 

 south-west side of the focal area there was a depression of at 

 least 5 ft. This tilt, or folding, as we may fairly call it, was also 

 accompanied by fracture of the rocks, showing itself by surface 

 rifts that ran for many miles north-east and south-west. 



Again, reference to the map will show that the line joining 

 Wellington and the epifocal area of the Cheviot earthquake of 

 November, 1901, is nearly parallel to the general axis of New 

 Zealand ; a great rift (called by Mr. A. McKay, Government Geo- 

 logist, "the Clarence fault") runs nearly in the same direction, 

 and it is quite probable that it indicates the existence of a deep 

 fault; The significance of this will be seen if we turn to the Wel- 

 lington seismogram of the Cheviot earthquake (fig. 3) ; it will be 

 observed that the mean position of the central or zero line after 

 the shock is nearer the lower edge of the paper — that is, nearer 

 the west — than it was before the shock : this shows that the 

 surface of the earth on which the column rests was tilted 

 through an angle of about 1*3 seconds towards the west or north- 



